1. Field of the Invention
The invention relates to image display sets in which the image produced by a cathode-ray tube is conveyed by means of an optical device to be displayed on a display screen. The invention relates more particularly to means to achieve the relative positioning between the cathode-ray tube and the optical device.
Among display sets requiring means of this kind to obtain positioning between the cathode-ray tube and the optical device, we might cite for example:
--television image projectors; PA0 --electronic visor devices in military aircraft; PA0 --landing-assistance devices in civilian and military aircraft.
In all these devices, the light carrying the image produced by the cathodoluminescent screen of the cathode-ray tube is picked up by the lens or lenses of an optical device, and then reflected by mirrors to be projected on a display screen. To preserve the qualities of the image produced by the cathode-ray tube, it is necessary to achieve accurate optical coupling between this tube and the optical device. This necessitates a perfect definition of the plane of the image (inside the cathode-ray tube) with respect to mechanical references that are used for the assembly.
2. Description of the Prior Art
A cathode-ray tube (CRT), incorporated in a display set of the type defined here above, constitutes an electronic component with a lifetime that may be shorter than that of the set in which it is mounted, and it should therefore be be easily replaced. To this end, the CRTs thus used are always positioned in a mounting shell or case, made of metal, having high-precision mechanical reference bearing surfaces. These mechanical reference bearing surfaces constitute mechanical references which, when the CRT and the optical device are being joined, taking the optical characteristics of the optical device into account, enable the position of this optical device to be defined with respect to the position of the plane of the image.
The plane of the image corresponds to the apparent plane of the luminophor, namely to the apparent plane of the cathodoluminescent screen. In a monochromatic CRT for example, the cathodoluminescent screen is foraged by a layer of luminophors. The layer of luminophors is generally deposited on an internal face of a transparent glass plate which closes a bulb with which it constitutes the vacuum-tight chamber of the CRT.
It must be noted that the image plane or apparent plane of the luminophor is different from the plane of the internal face of the plate, for the "optical thickness" Eo of the plate is related to its real thickness Er by the relationship Eo=Er/n, n being the refractive index of the glass forming the plate. Consequently, the image plane is located slightly before the internal face, inside the plate. Since the real thickness of the plate is known, the position of the image plane may be defined from the external face of the plate, and reference markers of this position may be put on the metal mounting case containing the CRT, these reference markers being constituted by the above-mentioned mechanical reference bearing surfaces.
In the case of a circular CRT, the mechanical reference bearing surfaces forming reference markers are often constituted, for example, by a cylindrical collar, plus a pin or a hole for the angular reference marking. In the case of a rectangular CRT, these mechanical reference bearing surfaces may be more complicated, and three fastening lugs may be necessary.
The CRT should therefore be fixed in a well-defined position in the metal mounting case. This is a position in which the CRT is held by resins that are poured or injected between the interior of this case and itself. This constitutes a relatively delicate coating operation which requires complex tools that should notably fulfil the functions of impervious sealing, holding the elements together, positioning the image plane with respect to the mechanical reference bearing surfaces and the observation, on the CRT screen, of the action of the deflection means which should be housed in the mounting case and around the CRT.
It is often observed, notably because of the varyingly dissymmetrical thrust forces that may be exerted at the time of the polymerization of the injected resins, that there are small differences in the position of the image plane, after the joining of the CRT and the mounting case.
The present invention relates to a display set of the kind defined here above, and its object is to reduce or even eliminate certain of the drawbacks related to the referencing of the position of the image plane and to the positioning of the optical device with respect to this image plane.